ri.1: rewrite ri man page

	* man/ri.1: update the (very outdated) ri man page:
	  * update document date
	  * fix document title formatting and volume name
	  * update descriptions and options to current ri --help text
	  * fix some mdoc formatting errors (missing escaping of `\',
	    wrong macro for bullet list items)
	  * various rewordings and other improvements

	improve man pages

	* man/ruby.1, man/erb.1, man/goruby.1, man/irb.1:
	  fix document title formatting and volume name,
	  improve "REPORTING BUGS" section: fix mdoc formatting error
	  (wrong macro for bullet list items), small rewordings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@62132 b2dd03c8-39d4-4d8f-98ff-823fe69b080e